Why Do Some LSB Dwarf Galaxies Have Lots Of Dark Matter While Some Seemingly Have None?

The paper does not consider whether the MOND external field effect (EFF) can adequately explain the departure from the mass discrepancy-acceleration relation that MOND codifies in the form of a phenomenological toy model gravity modification.

We use a sample of galaxies with high-quality rotation curves to assess the role of the luminous component ("baryons") in the dwarf galaxy rotation curve diversity problem. 

As in earlier work, we find that the shape of the rotation curve correlates with baryonic surface density; high surface density galaxies have rapidly-rising rotation curves consistent with cuspy cold dark matter halos, slowly-rising rotation curves (characteristic of galaxies with inner mass deficits or "cores") occur only in low surface density galaxies. 

The correlation, however, seems too weak in the dwarf galaxy regime to be the main driver of the diversity. In particular, the observed dwarf galaxy sample includes "cuspy" systems where baryons are unimportant in the inner regions and "cored" galaxies where baryons actually dominate the inner mass budget. 

These features are important diagnostics of the viability of various scenarios proposed to explain the diversity, such as (i) baryonic inflows and outflows; (ii) dark matter self-interactions (SIDM); (iii) variations in the baryonic acceleration through the "mass discrepancy-acceleration relation" (MDAR); or (iv) non-circular motions in gaseous discs. A reanalysis of existing data shows that MDAR does not hold in the inner regions of dwarf galaxies and thus cannot explain the diversity.

Together with analytical modeling and cosmological hydrodynamical simulations, our analysis shows that each of the remaining scenarios has promising features, but none seems to fully account for the observed diversity. The origin of the dwarf galaxy rotation curve diversity and its relation to the small structure of cold dark matter remains an open issue.

Isabel M.E. Santos-Santos, et al., "Baryonic clues to the puzzling diversity of dwarf galaxy rotation curves" (November 20, 2019) (submitted to MNRAS).

The conclusion in the body text states:

SUMMARY AND CONCLUSIONS

Dwarf galaxy rotation curves are challenging to reproduce in the standard Lambda Cold Dark Matter (LCDM) cosmogony. In some galaxies, rotation speeds rise rapidly to their maximum value, consistent with the circular velocity curves expected of cuspy LCDM halos. In others, however, rotation speeds rise more slowly, revealing large “inner mass deficits” or “cores” when compared with LCDM halos (e.g., de Blok 2010). This diversity is unexpected in LCDM, where, in the absence of modifications by baryons, circular velocity curves are expected to be simple, self-similar functions of the total halo mass (Navarro et al. 1996b, 1997; Oman et al. 2015). We examine in this paper the viability of different scenarios proposed to explain the diversity, and, in particular, the apparent presence of both cusps and cores in dwarfs. 

In one scenario the diversity is caused by variations in the baryonic contribution to the acceleration in the inner regions, perhaps linked to rotation velocities through the “mass discrepancy acceleration relation” (MDAR; McGaugh et al. 2016). In agreement with previous work, we show here that the inner regions of many dwarf galaxies deviate from such relation, especially those where the evidence for “cores” is most compelling. We conclude that the MDAR does not hold in the inner regions of low-mass galaxies and, therefore, it cannot be responsible for the observed diversity. 

A second scenario (BICC; “baryon-induced cores/cusps”) envisions the diversity as caused by the effect of baryonic inflows and outflows during the formation of the galaxy, which may rearrange the inner dark matter profiles: cores are created by baryonic blowouts but cusps can be recreated by further baryonic infall (see; e.g., Navarro et al. 1996a; Pontzen & Governato 2012; Di Cintio et al. 2014a; Tollet et al. 2016; Ben´ıtez-Llambay et al. 2019). 

A third scenario (SIDM) argues that dark matter self-interactions may reduce the central DM densities relative to CDM, creating cores. As in BICC, cusps may be re-formed in galaxies where baryons are gravitationally important enough to deepen substantially the central potential (see, e.g., Tulin & Yu 2018, for a recent review). 

We have analyzed cosmological simulations of these two scenarios and find that, although they both show promise explaining systems with cores, neither reproduces the observed diversity in full detail. Indeed, both scenarios have difficulty reproducing an intriguing feature of the observed diversity, namely the existence of galaxies with fast-rising rotation curves where the gravitational effects of baryons in the inner regions is unimportant. They also face difficulty explaining slowly-rising rotation curves where baryons actually dominate in the inner regions, which are also present in the observational sample we analyze. 

We argue that these issues present a difficult problem for any scenario where most halos are expected to develop a sizable core and where baryons are supposed to be responsible for the observed diversity. This is especially so because the relation between baryon surface density and rotation curve shape is quite weak in the dwarf galaxy regime, and thus unlikely to drive the diversity. We emphasize that, strictly speaking, this conclusion applies only to the particular implementations of BICC and SIDM we have tested here. These are by no means the only possible realizations of these scenarios, and it is definitely possible that further refinements may lead to improvements in their accounting of the rotation curve diversity. 

Our conclusions regarding SIDM may seem at odds with recent work that reports good agreement between SIDM predictions and dwarf galaxy rotation curves (see; e.g., the recent preprint of Kaplinghat et al. 2019, which appeared as we were readying this paper for submission, and references therein). That work, however, was meant to address whether observed rotation curves can be reproduced by adjusting the SIDM halo parameters freely in the fitting procedure, with promising results. Our analysis, on the other hand, explores whether the observed galaxies, if placed in average (random) SIDM halos, would exhibit the observed diversity. Our results do show, in agreement with earlier work, that SIDM leads to a wide distribution of rotation curve shapes. However they also highlight the fact that outliers, be they large cores or cuspy systems, are not readily accounted for in this scenario, an issue that was also raised by Creasey et al. (2017). Whether this is a critical flaw of the SIDM scenario, or just signals the need for further elaboration, is still unclear. 

We end by noting that the rather peculiar relation between inner baryon dominance and rotation curve shapes could be naturally explained if non-circular motions were a driving cause of the diversity. For this scenario to succeed, however, it would need to explain why such motions affect solely low surface brightness galaxies, the systems where the evidence for “cores” is most compelling. Further progress in this regard would require a detailed reanalysis of the data to uncover evidence for non-circular motions, and a clear elaboration of the reason why non-circular motions do not affect massive, high surface brightness galaxies. Until then, we would argue that the dwarf galaxy rotation curve diversity problem remains, for the time being, open.